The present invention relates to electrical conductivity in metal oxide ceramic materials and the creation of electrical conductivity in a normally non-conductive material. It has application in providing electrical conductivity across a layer of ceramic material. In a particular application the layer of metal oxide ceramic is adhered to a metal plate and this has special application in components of solid oxide fuel cells.
Alumina is well known as an electrical insulator and as a material which is physically and chemically stable at high temperatures. Its electrical properties are put to good use in many high temperature applications where electrical isolation is desired. However, it would be useful in many applications to have a material which has the high temperature stability of alumina while also having good electrical conductivity. It would be particularly useful if thin layers or sheets of alumina or other metal oxide ceramics could be made electrically conductive through the ceramic in selected locations.
It has now been found in one embodiment that the application of silver metal, in any of a variety of forms, to the surface of a fully dense body of metal oxide ceramic such as alumina or chromia followed by a sustained heat treatment at temperatures in the range 75xc2x0 C.-97xc2x0 C. or above, can cause the ceramic to develop electrically conductivity, especially in the immediate vicinity of the silver. Alternatively, the silver may be applied to a surface of a metal substrate on which the fully dense body of metal oxide is caused to form. The conductivity so imparted to the ceramic may be a volume effect, that is, the conductivity may be imparted both laterally and through the thickness of the ceramic body. The body may be a layer, sheet, film or thin plate. However, with a body having a very small thickness, the effect may be principally through the ceramic.
Thus according to a first aspect of the present invention there is provided a body of fully dense metal oxide ceramic material which has been rendered electrically conductive through its thickness by the incorporation of silver into the material.
Preferably the ceramic material is alumina, but it may alternatively be, for example, chromia, or alumina-rich or chromia-rich ceramic.
Preferably the metal oxide ceramic material has a thickness of no more than 1 mm, more preferably no more than 10 xcexcm.
In a preferred embodiment, the metal oxide ceramic material is a layer on a substrate. The substrate may be of any suitable material on which the layer can be provided. The layer of metal oxide ceramic material can be applied by any suitable means to the substrate, or it may be grown on the substrate, for example as in the case of a self-aluminising steel, that is a steel having an aluminium content of greater than 4.5 wt %.
In a solid oxide fuel cell the electrolyte, anode and cathode are normally ceramic materials. However, the surrounding structural components of a fuel cell stack may be of any material which can provide the required electrical conduction properties and structural strength to the stack assembly, at the temperatures necessary for operation of the fuel cell, for example in excess of 700xc2x0 C. These components may be made of a ceramic or metal capable of handling the conditions. Some of these components, for example bipolar plates (also known as interconnect plates), are required to provide electrical connection between adjacent fuel cells. Sophisticated electrically conductive ceramics have been developed for this purpose but these materials are expensive, mechanically fragile and are poor thermal conductors when compared with many metals capable of handling the conditions.
The operating conditions in a solid oxide fuel cell are very severe on most metals, causing them to degrade via loss of mechanical strength, oxidation or other form of corrosion, distortion, erosion and/or creep. Various heat resistant metals have been developed to cope with many of these forms of degradation. Most such metals are alloys based on iron or nickel with substantial additions of chromium, silicon and/or aluminium, plus, in some alloys, more expensive elements such as cobalt, molybdenum and tungsten. Chromium based metals are also available.
The significant feature of all heat resistant steels is the oxide layer which is formed when the steel is exposed to mildly or strongly oxidizing conditions at elevated temperatures. They all form tight, adherent, dense oxide layers which prevent further oxidation of the underlying metal. These oxide layers are composed of chromium, aluminium or silicon oxides or some combination of these depending upon the composition of the steel. They are very effective in providing a built-in resistance to degradation of the underlying steel in high temperature oxidizing conditions.
However, while this feature is used to great advantage in many applications, the presence of the oxide layer is highly deleterious to the use of these steels in key components of solid oxide fuel cells. These oxides, especially those of silicon and aluminium, are electrically insulating at all temperatures and this is a major problem for those fuel cell components which must act as electrical current connectors. For these heat resisting steel to be useful for electrical conducting components in fuel cells, it is imperative that the insulating effect of the oxide layer be alleviated at least in selected locations.
According to a second aspect of the invention, there is provided a component formed of steel having a surface layer of alumina, chromia or alumina-rich or chromia-rich fully dense ceramic, said layer having been rendered electrically conductive through its thickness by the incorporation of silver into the layer.
The ceramic layer protects the underlying metal from chemical interactions while the electrical conductivity provided by the silver allows it to provide electrical contact with the underlying metallic component.
The silver may be incorporated into the layer as the layer is formed on the steel or after the layer has been formed on the steel. Preferably, the layer is formed by surface oxidation of the steel, for example as in the case of a self-aluminising steel.
For fuel cell and other applications, an advantage of the present invention is that a material such as alumina which is universally renowned for its excellent thermal and electrically insulating properties, as well as its chemical inertness, can have one of these three properties reversed without impairing the other two. The invention can provide, with alumina, a material which is still an excellent refractory material and inert in nearly all environments, but which is electrically conductive at least in selected positions. This is of special significance for various connections required in fuel cell assemblies. The effect has been found to be durable over long periods of time and over the full temperature range required for solid oxide fuel cell operation. The invention has been used to advantage to convert otherwise highly insulating alumina coated metal bipolar plates to conducting plates which can be used to collect current from operating fuel cells. The conductivity can be used as a sole means of current collection or used as a safeguard/backup in case a prime current collector mechanism fails.
The mechanism by which the silver migrates into or occurs in the metal oxide ceramic is not fully understood at this time. However, it is believed that the electrical conductivity is provided by the silver extending along grain boundaries of ceramic material. Incorporating the silver into the ceramic material can be achieved by heating the silver-containing material in contact with the ceramic material or with a substrate on which the ceramic material is formed.
According to a third aspect of the invention there is provided a method of providing electrical conductivity through a body of fully dense metal oxide ceramic material including placing a silver-containing material into contact with the ceramic material and heating the ceramic and silver-containing materials in contact with each other to at least 750xc2x0 C. such that silver migrates from said silver-containing material into said layer of metal oxide ceramic material and creates electrically conductive pathways through the ceramic material.
The atmosphere in which the method is performed does not appear to be important and is conveniently air. The method is conveniently performed at atmospheric pressure.
According to a fourth aspect of the invention there is provided a method of forming a steel component with a heat-resistant and electrically conductive surface layer, said method including selecting a steel which forms an alumina, chromia or alumina-rich or chromia-rich fully dense surface layer in oxidizing atmosphere,placing a silver-containing material in contact with the surface of the steel, heating the steel and silver-containing material to at least 750xc2x0 C. in an oxidizing atmosphere to cause said surface layer to form on the steel and to cause silver from said silver-containing material to occur in and to create electrically conductive pathways through the layer.
Preferably the steel used in the method of the fourth aspect of the invention has an aluminium content of above 4.5 wt %.
Preferably, the heating step in the methods of the invention is to at least 800xc2x0 C., more preferably at least 850xc2x0 C., even more preferably at least 900xc2x0 C. and most preferably at least 950xc2x0 C. It is believed that while the effect of the silver imparting electrical conductivity to the metal oxide ceramic material will occur at 750xc2x0 C., or even less, the rate of the effect occurring is very slow at this temperature and increases with increasing temperature. The effect occurs especially quickly when the silver is in a liquid state,
The silver-containing material is preferably at least commercially pure silver, but it may be an alloy or otherwise contain selected impurities which are not severely detrimental to the effect of imparting electrical conductivity to a metal oxide ceramic material. Such impurities or alloying elements may include one or more of the noble metals, Sn, Cu and Ni.
The silver-containing material may be in sheet, mesh, paste or other appropriate form. The silver-containing material may be provided on a substrate of a type which is acceptable to the end result.